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Nonmetallic Sheathed Cable

Throughout the vast majority of the U.S.A., nonmetallic sheathed cable (NM cable) has been the most common type of residential wiring system since about the early 1950’s.  And it’s had a pretty good track record of success.  It consists or two or more insulated wires along with a bare ground wire, wrapped in paper and then contained inside a thermoplastic outer sheathing.  It’s often called Romex®, but that’s a brand name and not all NM cable is Romex®.  It comes in large spools of different wire sizes, it’s easy to install, it’s easy to cut to length, and it goes up pretty quickly.

Despite its long history of successful usage throughout most of the country, in much of the Chicagoland area this type of wiring isn’t allowed.  Chicago and many of the surrounding suburbs require that wires be pulled through some type of raceway, which usually is electrical metallic tubing (EMT) and normally called “conduit”.

So when you see NM cable in a part of Chicagoland where it’s not allowed, what should you think?  And what should you do?

The first thing to note is that if this issue is coming up when you’re buying a house then it’s likely to come up again when you sell.  So keep that in mind.

The biggest problem with NM cable in Chicagoland is that it’s often installed not by a good qualified electrician but by a handyman or homeowner.  In this case it’s not the material that’s in question but the installation methods.  When it’s exposed, NM cable needs to be run closely along the surfaces of the building finish to provide support and protection.  It needs to be supported at least every 4.5 feet and secured within 12 inches of its ends.  Where the cable enters any type of panel enclosure or junction box it needs to be clamped to the box.  These are very common defects when NM cable is installed by an amateur.

This house is in Chicago so there shouldn’t be any NM cable. Still, this is run closely along the building surfaces, it’s supported properly, and it’s clamped into this junction box for the light. It’s installed well.

This cable isn’t clamped to the box. Not done by a licensed electrician.

NM cable can only be used inside.  It can’t be used outside or exposed to sunlight.  It can’t be buried underground (there’s a special kind of cable for that) or encased in concrete or plaster.

NM cable shouldn’t be used outside. No self-respecting electrician did this.

NM can’t be used with a plug, making it into an extension cord.  It isn’t designed to be bent back and forth many times like an extension cord is.

A double whammy — NM cable run through a concrete wall and used with a plug as a glorified extension cord. Not done by a good electrician.

In an attic NM cable needs to be protected from physical damage.  Often times the cable is just run across the tops of the framing members, right where you want to step.  NM cable needs to be protected within six feet of any attic access opening, and if the attic has a permanent ladder or stairs then it needs protection throughout the attic up to a height of seven feet.

When NM cable is run in an unfinished basement along the bottoms of floor joists it needs to be attached to a running board (usually a 1×4 board fastened to the bottoms of the joists).  Or it can be installed through holes bored in the middle of each floor joist.

Cable that’s run along the bottoms of floor joists is susceptible to physical damage. First a 1×4 running board should be installed, and then the cable is attached to that.

NM cable is much more susceptible to damage from nails than is conduit.  So when NM cable is run through a wall stud hole that’s less than 1-1/4 inch from the front edge it needs to be protected with a steel strike plate to stop an errant nail from piercing the wire.

And dealing with the equipment grounding conductor (the ground wire) is quite a bit different with NM cable than with conduit, so that has to be taken into account.

So if the NM cable in your house is installed well then it’s likely to not pose a hazard, even though it wasn’t installed under a building permit like it should have been.  But if it has any of these defects described above then the risk of problems rises greatly.  And that can lead to shock, electrocution, fire, or other problems.  Electrical wiring is not the place you want to see amateur workmanship in your

Electrical grounding

Electrical grounding is a very misunderstood topic.  Here’s a quick description of the major facts about electrical grounding.

There are two distinct grounding systems in your house.  First we have the grounding electrode system.  This is the system that connects one half of the electrical system directly to the ground.  We call this have of the system the “neutral” wire.

Grounding Electrode System

In the USA we use a grounded electrical system.  This means that what we call the neutral wire is physically connected to the ground at various points in the system.  It’s grounded at the utility transformer, at the electric meter on the side of your hose (through a buried ground rod), and at the metal water pipe coming into your house from the street.

Not all countries use a grounded system like this.  But since 1913 our National Electrical Code has required this.  It helps to protect us from high-voltage crossover problems at the transformer, and nearby lighting strikes.  It helps to discharge static electricity, and it provides protection in case the service neutral wire coming to the house from the utility ever gets broken.

A grounding electrode is a piece of metal that creates this direct connection to the ground.  In a typical house the grounding electrodes consist of the buried metal water pipe and a ground rod driven into the ground near the electric meter.

Any wire that helps to create this direct connection to the earth is called a grounding electrode conductor.  You should have such a grounding electrode conductor going from your main electrical panel over to where the metal water pipe enters your house from the street.

So the grounding electrode system connects much of the metal in your house together with the neutral wires and connects it to the ground.  Basically it helps to protect us from electricity that originates outside of the house – like lighting or transformer problems.

Equipment Grounding System

Equipment grounding is a very different part of the electrical system, although they are connected together.  When you plug in a device with a 3-prong plug that third prong is providing equipment grounding.  Equipment grounding is intended to protect us from electricity that comes from the appliances we use.  And the most important thing to understand about equipment grounding is that it has nothing to do with the ground, or the earth, or the dirt beneath our feet.

So the ground rod or metal water pipes mentioned as part of the Grounding Electrode System has nothing to do with equipment grounding.  The National Electrical Code (NEC) forbids using the earth as an equipment grounding path. NEC 250.4(A)(5) “. . . . The earth shall not be considered as an effective ground-fault current path.”

Here’s a schematic showing how electricity is wired from the utility transformer into an appliance in your house.

Electricity wants to complete a path, so in the image above electricity wants to complete the circuit from point A to point B (in the utility transformer, where the neutral is grounded through a driven copper ground rod).  The schematic below shows what happens when we close the switch (just to the right of the appliance in the schematic above) and turn on the appliance.  You can follow the electrical path — follow the little red lightning bolts.  Electrical current starts at Point A and goes into the house through the meter (where the neutral is connected to the ground through a driven copper ground rod), into the electrical panel (where the neutral is connected to the ground through metal water pipes), through a main breaker and a branch circuit breaker, and out to the appliance. The current goes through the appliance load and then returns through the neutral wire out to Point B at the transformer.

This is the way things were before about 1960, when there was no equipment ground.

Eventually we learned that an effective equipment grounding system could help to protect people, so we instituted that requirement. Here’s how we did NOT do that. This is a schematic showing a ground connection from the appliance case directly into the ground.

Now suppose that the hot wire has some torn insulation and the wire makes contact with the metal case of the appliance. That’s a ground fault. Now the case of the appliance is at 120 volts to ground and if you touch it you could be shocked. Where is the electrical current going? Remember that electricity wants to make a circuit, so the current wants to go back to point B in the utility transformer.  Follow the little red lightning bolts again to see the path that electrical current takes.

There are two paths for current to return to the transformer. One is through your hand, through your heart (that’s not good!), into the ground, and then over to the ground rod at the utility transformer. Another path is through the appliance’s ground rod and over to the utility transformer. Both paths have a lot of resistance, so neither will be favored much and current will flow through both paths. (When talking about alternating current the proper term is impedance, not resistance. But I’m going to keep using the term resistance.) So a lot of current will flow through you, certainly enough to kill you. But the resistance of the earth is too high so not enough current will flow to trip off the breaker. So you’re standing here as part of an electrical path that’s sending current through your body. Terrible.

Here’s another schematic showing the proper equipment grounding configuration. There’s simply a ground wire going from the appliance case back to the neutral bus bar in the main electrical panel.

Now if there’s a ground fault and you touch the appliance case, there are again two paths for current to take to get back to the utility transformer. One is through you and into the ground and back to the transformer. That’s a lot of resistance.

But another path is through the ground wire, back to the electrical panel, and then back out to the transformer through the service neutral. And this path has no load at all on it, so it has very low resistance. With low resistance this path will take most of the current — a lot of current. Electrical current favors the path of least resistance. Enough current will flow through the ground wire to quickly trip off the branch circuit breaker. And that’s what you really want — you want the breaker to trip off and de-energize the appliance.

This is the way that equipment grounding keeps us safe in the U.S.

Reversed Polarity Electrical Receptacles

A typical 120-volt electrical circuit has a hot wire and a neutral wire.  The hot wire has 120 volts to ground and so it can easily shock you.  But the neutral wire is physically connected to the ground, so it shouldn’t have any voltage present.  This makes it a lot safer — but there are plenty of things that can go wrong, so please don’t ever touch any wire.

Electrical devices are designed to take advantage of this difference between the hot and neutral wires.  Any appliance that has a switch is going to be safer if, when the switch is off, there’s no voltage inside the appliance.

The way to be sure that there’s no voltage inside the appliance when the switch is off is to put the switch on the hot wire.  So when the switch is off only the neutral wire is still connected back to the electrical panel, and since the neutral wire is connected to the ground there’s no voltage difference to push electrical current.

Here’s a simple schematic of an appliance showing the issue.  The switch is on the hot side of the appliance, so that when it’s switched off, the load is only connected to the neutral wire, which is connected to the ground and so it should have zero electrical potential.  The appliance is much safer this way.

A typical appliance plugged into a receptacle with correct polarity. When the switch is off there are no live electrical connections in the appliance.

One blade of the plug and one slot of the receptacle are wider (this is the neutral side), so that the plug can only be inserted with one orientation. This ensures proper polarization — as long as the receptacle is wired correctly.

To make sure this happens, we use polarized plugs and polarized receptacles.  Plugs on devices that need to be polarized have one blade wider than the other.  And all receptacles have one slot wider than the other, so that the plug can only physically be inserted in one way.  This makes sure that the appliance sees the proper polarity.

Of course the only way this works is if the wires feeding into the electrical receptacle are connected in the proper way, with the hot wire attached to its proper location on the receptacle (the narrow slot), and the neutral wire connected to its proper location (the wide slot).  If the receptacle is wired backwards, then we have a condition known as reversed polarity.

Here’s our appliance schematic showing what happens if it’s plugged into a receptacle with reversed polarity.  Now even when the switch is off the load inside the appliance is connected to the hot side of the wiring and all the electrical components are live all the time.  This dramatically increases the chance of electrical shock.

An appliance plugged into a receptacle with reversed polarity. Even when the switch is off all of the components of the appliance are energized.

One very important example of this situation is a lamp.  Consider the lamp holder — where you screw in the light bulb.  You want the threads of the shell (green arrow in picture below) to always be on the neutral side, because it’s fairly easy to touch this accidentally. You want the button at the bottom (red arrow) to always be on the hot side, because it’s hard to touch it.  If a lamp is plugged into a receptacle with reversed polarity then the shell of the lamp holder is always going to be live, even when the lamp is switched off, and so it’s easy to be shocked when you change the bulb.

If a receptacle has reversed polarity it’s probably just because the wires at the receptacle are reversed.  This is an easy fix.  But it might be the case that the reversal was done somewhere farther back upstream.  An electrician needs to find the source of this problem.